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Saturn’s rings, younger than dinosaurs?

Some of Saturn‘s icy moons may have been formed after many dinosaurs roamed the Earth. New computer modeling of the Saturnian system suggests the rings and moons may be no more than 100 million years old.

Saturn hosts 62 known moons. All of them are influenced not only by the gravity of the planet, but also by each other’s gravities. A new computer model suggests that the Saturnian moons Tethys, Dione and Rhea haven’t seen the kinds of changes in their orbital tilts that are typical for moons that have lived in the system and interacted with other moons over long periods of time. In other words, these appear to be very young moons.

“Moons are always changing their orbits. That’s inevitable,” Matija Cuk, principal investigator at the SETI Institute and one of the authors of the new research, said in a statement. “But that fact allows us to use computer simulations to tease out the history of Saturn’s inner moons. Doing so, we find that they were most likely born during the most recent 2 percent of the planet’s history.”

The age of Saturn’s rings has come under considerable debate since their discovery in the 1600s. In 2012, however, French astronomers suggested that some of the inner moons and the planet’s well-known rings may have recent origins. The researchers showed that tidal effects — which refer to “the gravitational interaction of the inner moons with fluids deep in Saturn’s interior,” according to the statement — should cause the moons to move to larger orbits in a very short time.

“Saturn has dozens of moons that are slowly increasing their orbital size due to tidal effects. In addition, pairs of moons may occasionally move into orbital resonances. This occurs when one moon’s orbital period becomes a simple fraction of another. For example, one moon could orbit twice as fast as another moon, or three times as fast. Once an orbital resonance takes place, the moons can affect each other’s gravity, even if they are very small. This will eventually elongate their orbits and tilt them from their original orbital plane. By looking at computer models that predict how extended a moon’s orbit should become over time, and comparing that with the actual position of the moon today, the researchers found that the orbits of Tethys, Dione and Rhea are “less dramatically altered than previously thought,” the statement said.

The moons don’t appear to have moved very far from where they were born. To get a more specific value for the ages of these moons, Cuk used ice geysers on Saturn’s moon Enceladus. The researchers assumed that the energy powering those geysers comes from tidal interactions with Saturn and that the level of geothermal activity on Enceladus has been constant, and from there, inferred the strength of the tidal forces from Saturn.

Using the computer simulations, the researchers concluded that Enceladus would have moved from its original orbital position to its current one in just 100 million years — meaning it likely formed during the Cretaceous period. The larger implication is that the inner moons of Saturn and its gorgeous rings are all relatively young. (The more distant moons Titan and Iapetus would not have been formed at the same time.)

“So the question arises — what caused the recent birth of the inner moons?” Cuk said in the statement. “Our best guess is that Saturn had a similar collection of moons before, but their orbits were disturbed by a special kind of orbital resonance involving Saturn’s motion around the sun. Eventually, the orbits of neighboring moons crossed, and these objects collided. From this rubble, the present set of moons and rings formed.” The research is being published in the Astrophysical Journal.

The age of Saturn’s rings has long proven controversial. Some researchers had thought the iconic features formed along with the planet about 4.5 billion years ago from the icy rubble left in orbit around it after the formation of the solar system. Others suggested the rings are very young, perhaps originating after Saturn’s gravitational pull tore apart a comet or an icy moon.

One way to solve this mystery is to weigh Saturn’s rings. The rings were initially made of bright ice, but over time have become contaminated and darkened by debris from the outer reaches of the solar system. A few years back, NASA’s Saturn-orbiting Cassini mission determined that the rings are only about 1 percent impure. If scientists could weigh Saturn‘s rings, they could estimate the amount of time it would take for them to accumulate enough contaminants to get 1 percent impure and thus calculate their age, lead study author Luciano Iess, a planetary scientist at the Sapienza University of Rome, told Space.com. [Saturn’s Glorious Rings in Pictures]

Iess and his colleagues relied on more Cassini data. Before the spacecraft plunged to its death into Saturn’s atmosphere in September 2017, it coasted between the planet and its rings and let their gravitational pulls tug it around. The strength of a body’s gravity depends on its mass, and by analyzing how much Cassini was pulled one way or the other during the “grand finale” phase of its mission, the mission team could measure the gravity and mass of both Saturn and its rings.

During six of Cassini’s crossings between Saturn and its rings at altitudes about 1,615 miles to 2,425 miles (2,600 to 3,900 kilometers) above the planet’s clouds, scientists monitored the radio link between the spacecraft and Earth. Much as how an ambulance siren sounds higher pitched as the vehicle drives toward you and lower pitched as it moves away, the radio signals would lengthen in wavelength as their source moved away Earth and shorten as their source moved toward it — an effect called the Doppler shift.

“I’m astonished by the fact that we were able to measure the velocity of a distant spacecraft 1.3 billion kilometers [807 million miles] away from Earth with an accuracy that is a hundredth or a thousandth the speed of a snail — a few hundreds of millimeters per second,” Iess said.

Previous estimates based on data from the Voyager flybys of Saturn suggested the rings’ mass was about 28 million billion metric tons. The new data from Cassini now suggests the rings’ mass is only about 15.4 million billion metric tons. (The largest asteroid, Ceres, has a mass of about 939 million billion metric tons.)

Cassini’s grand finale also revealed key details about the internal structure of Saturn. For example, it found that jet streams seen around Saturn’s equator — the strongest measured in the solar system, with winds of up to 930 mph (1,500 km/h) — extend to a depth of at least 5,600 miles (9,000 km), rotating a colossal amount of mass around the planet about 4 percent faster than the layer below it.

“The discovery of deeply rotating layers is a surprising revelation about the internal structure of the planet,” Cassini project scientist Linda Spilker at NASA’s Jet Propulsion Laboratory in Pasadena, California, who did not participate in the study, said in a statement. “The question is, What causes the more rapidly rotating part of the atmosphere to go so deep, and what does that tell us about Saturn’s interior?”

The new findings also suggest that Saturn’s rocky core is about 15 to 18 times the mass of Earth, similar to prior estimates.

The scientists detailed their findings online Jan. 17 in the journal Science.